The subject matter disclosed herein relates to multi-reflection optical systems, and more particularly to collector optics for lithography and imaging applications, and to their fabrication.
A well known optical design for X-ray applications is the type I Wolter telescope. The optical configuration of type I Wolter telescopes consists of nested double-reflection mirrors operating at grazing incidence.
More recently, a variation of the type I Wolter design already proposed for other applications, in which the parabolic surface is replaced by an ellipsoid, has found application for collecting the radiation at 13.5 nm emitted from a small hot plasma used as a source in Extreme Ultra-Violet (EUV) microlithography, currently considered a promising technology in the semiconductor industry for the next generation lithographic tools. Here, there is a performance requirement to provide a near constant radiation energy density or flux across an illuminated silicon wafer target at which an image is formed. The hot plasma in EUV lithography source is generated by an electric discharge (Discharge Produced Plasma or DPP source) or by a laser beam (Laser Produced Plasma or LPP source) on a target consisting of Lithium, Xenon, or Tin, the latter apparently being the most promising. The emission from the source is roughly isotropic and, in current DPP sources, is limited by the discharge electrodes to an angle of about 600 or more from the optical axis. EUV lithography systems are disclosed, for example, in US2004/0265712A1, US2005/0016679A1 and US2005101 55624A1.
A simplified block diagram of a known EUV lithography system is shown in FIG. 1. The ultra-violet source 102 is normally a hot plasma, the emission of which is collected by the collector 104 and delivered to an illuminator 106. The latter illuminates a mask or reticle 108 with the pattern to be transferred to the wafer 110. The image of the mask or reticle is projected onto the wafer 110 by the projection optics box 112.
Presently, the most promising optical design for collectors 104 is based on nested Wolter I configuration, as illustrated in FIG. 2. Each mirror 200 is a thin shell consisting of two sections (surfaces) 202, 204: the first one 202, closer to the source 102 is a hyperboloid whereas the second 204 is an ellipsoid, both with rotational symmetry, with a focus in common.
The light source 102 is placed in the focus of the hyperboloid different from the common focus. The light from the source 102 is collected by the hyperbolic section 202, reflected onto the elliptic section 204 and then concentrated to the focus of the ellipsoid, different from the common focus, and known as intermediate focus (IF) 206.
From an optical point of view, the performance of the collector 102 is mainly characterized by the collection efficiency and the far field intensity distribution. The collection efficiency is the ratio between the light intensity at intermediate focus 206 and the power emitted by the source 102 into half a sphere. The collection efficiency is related to the geometry of the collector 104, to the reflectivity of each mirror 200, to the spatial and angular distribution of the source 102, to the optical specifications of the illuminator. The far field intensity distribution is the 2D spatial distribution of the light intensity beyond the intermediate focus 206 at distances that depends on the illuminator design, but that are normally of the same order of magnitude as the distance between the source 102 and intermediate focus 206.
The collector 104 is normally used in conjunction with a hot plasma source 102. Thus, the thermal load from UV radiation on the collector 104 is very high and a proper cooling system is required. The cooling system is positioned on the back surface of each mirror 200 in the shadow area that is present on the back side of both the elliptical section 204 and the hyperbolic section 202 (see FIG. 2).
Referring to FIG. 3, in the design of a Wolter I mirror the hyperbolic 202 and the elliptical section 204 has a focus in common 304 that lays on the optical axis 302 (i.e. the line through the source focus 102 and the intermediate focus 206). This condition introduces a constraint in the design of the mirror 200, 200′ with the consequence that the designer has one degree of freedom (one real parameter, corresponding to the position of the common focus 304 on the optical axis 302) for each mirror. The resulting total number of degrees of freedom is further reduced by the system specification for the whole collector 104, by manufacturing requirements, etc. It is then possible that, in order to satisfy all the requirements and boundary conditions, the design of the collector is not fully optimized in terms of optical performance.
By way of example, FIG. 4 and Table A.1 show the optical layout and prescriptions of a Wolter I collector 104 designed for the following specifications:
Distance between source 102 and IF 206: 1500 mm
Maximum numerical aperture at IF: 0.139 (8°)
Minimum distance between source 102 and optics (104): 110 mm
Mirror thickness: 2 mm—Number of nested mirrors 200, 200′: 7
TABLE A.1Reference Wolter designHyperbolaEllipseMirror radii [mm]Radius ofRadius ofEllipse-ConiccurvatureConiccurvaturehyperbolaMirror #Constant[mm]Constant[mm]MaximumjointMinimum1−1.010193852.1365−0.998529111.413736.224234.323924.64182−1.017385363.6308−0.997556712.349446.710544.173131.52023−1.029148596.0526−0.995998793.850559.823856.472640.05284−1.048418269.9610−0.993504916.258276.319371.912950.65125−1.0803521716.2846−0.9894947210.142797.264691.456063.83626−1.1346993026.6371−0.9829683416.4983124.2702116.522480.27407−1.2320721244.0419−0.9720945927.1823159.9860149.3855100.8206
The design of FIG. 4 collects the light from the source 102 up to an angle of 55.5°. The collection efficiency of the collector 104 shown in FIG. 4 calculated for a point source and assuming a Ruthenium coating with theoretical reflectivity is 27.7% with respect to 2π sr emission.
Where comparative performance data are given herein for collector designs, these are relative to the design of FIG. 4.
FIG. 5 shows the grazing incidence angle on both the hyperbolic section 202 and elliptical section 204 as a function of the emission angle for the Wolter I collector 104 of FIG. 4. It can be noted that the grazing incidence angle on the hyperbolic section 202 is always greater than the grazing incidence angle on the elliptical section 204. The consequence of this difference is a decrease of the efficiency of the collector 104 since the maximum optical transmission is achieved when the two angles are equal.
The purpose of the collector 104 in EUV sources is to transfer the largest possible amount of in-band power emitted from the plasma to the next optical stage, the illuminator 106, of the lithographic tool 100 (see FIG. 1), with the collector efficiency being as defined hereinabove. For a given maximum collection angle on the source side, the collector efficiency is mainly determined by collected angle and by the reflectivity of the coating on the optical surface of the mirrors.
A problem with known systems is that that collector efficiency is significantly lower than it might be since the reflectivity of the coating is not exploited in the most efficient way. Any improvement in the collector efficiency is highly desirable.
A further problem is that, with the collector efficiencies available, there is imposed the need to develop extremely powerful sources, and to have high optical quality and stability in the collector.
A further problem is that the number of degrees of freedom in the design of each mirror is limited.
A further problem is that the collector lifetime may be relatively short due to exposure to extremely powerful sources.